1. Field of the Invention
The present invention relates to an apparatus for determining the activity of an ion (pIon) in a liquid. The apparatus comprises a measuring circuit including an ion sensitive field effect transistor (ISFET), a reference electrode, an amplifier, a temperature sensor, a memory, and a computing circuit. The measuring circuit provides a form of temperature compensation by maintaining two of three parameters for controlling the operation of the ISFET, namely V.sub.gs (gate-source potential), V.sub.ds (drain-source potential) and I.sub.D (drain-source current) at a constant value, and the third parameter is used for determining the ion activity.
2. Description of the Prior Art
Apparatus for determining ion activity in a liquid are known in the art. For example, British patent application No. 2,077,439 describes advantages which can be obtained with the use of an ISFET for the measurement of ion activities in a liquid. However, this patent application points out that an accurate measurement of the ion activity is hampered by the fact that the gate potential and the potential of the reference electrode are affected by the temperature (i.e. differences or changes in temperature). Also the degree to which these potentials are affected by the temperature varies from ISFET to ISFET.
When taking into consideration the variation in characteristic of the ISFET and the teachings of this British patent application, as a result of change in temperature, the following equation can be obtained: EQU V=f(c)+.differential.Eg/.sub..differential.T +.differential.Es/.sub..differential.T -.differential.E.sub.r /.sub..differential.T
in which V is the voltage measured, f(c) is the parameter to be determined, which is a function of the ion activity, and the other terms represent the variation of, respectively, the gate potential, the potential of the source relative to the gate potential, and the potential of the reference electrode with the temperature.
In utilizing this equation, the drain-source current is set at a predetermined value such that the temperature effects related to the terms given in the above equation cancel out. According to the British patent application, setting of the drain-source current at the selected value can be achieved by means of one or more series-connected fixed and variable resistors.
In a preferred embodiment of the apparatus disclosed in British patent application No. 2,077,439, the apparatus is provided with two mutually electrically coupled ISFETs to correct for changes in temperature. A correction of the pIon sensitivity with temperature is not, however, possible with this preferred embodiment.
In U.S. Pat. No. 4,267,504 it is stated that, in measuring a magnitude by means of a field effect transistor, neutralization of the temperature effect on the results of the measurement can be achieved by means of a second, temperature-sensitive element provided adjacent to the field effect transistor to obtain a separate temperature indication. At the same time, however, this patent states with regard to this feature that accurate and reliable measurement cannot be achieved in this way because it is never certain whether the two temperature-sensitive elements are always at the same temperature and whether they react to temperature variations in the same way. As will be described in greater detail hereinafter the apparatus of the present invention provides a different solution to the problem of compensating for temperature changes to obtain reliable and accurate measurements than are proposed in U.S. Pat. No. 4,267,504.
There is also proposed a scheme for temperature compensation of an ion sensing field effect transistor in German published patent application no. DE 3,144,459 which scheme is different than the temperature compensation scheme of the present invention.
As will be described in greater detail hereinafter, the present invention provides an apparatus in which the effect of the temperature on the result of an ion measurement is eliminated in a manner which is different than the temperature compensation schemes proposed in the prior art references referred to above and which is more flexible, i.e. one offering more options and better possibilities than the previously proposed temperature compensation schemes.